The present invention relates to a transmission device, a power output device, and a control method of the power output device, and more particularly to a transmission device that is built into a power output device outputting power to a drive shaft in combination with a power source and that includes an automatic transmission that changes a shift speed by switching the engagement state of at least one friction engagement element and transmits power from the power source to the drive shaft, a power output device that outputs power to a drive shaft, and a control method of the power output device.
Conventionally, as a type of the power output device described above, there has been proposed a device in which, when an upshift command of an automatic transmission has been issued, the torque transmission capacity of a clutch (friction element) to be engaged is gradually increased, then a pre-torque down operation is performed for reducing the torque of an engine before starting an inertia phase, and after starting the inertia phase, the torque down operation of the engine is performed again, thereby changing the shift speed (refer to Patent Document 1, for example). It is stated that this device can not only suppress a shift shock but also reduce the heat load of the clutch by performing the pre-torque down of the engine before starting the inertia phase.    [Patent Document 1] Japanese Patent Application Publication No. JP-A-2008-51268